The microcomputer system having the microcomputer and the storage arrangement constitutes, for example, one part of a control unit for a motor vehicle. A control unit of this type controls various functions in a motor vehicle, for example, the internal combustion engine, the transmission, the brake and power train, the driving-dynamics control system, etc. The control unit conventionally has a microcomputer that has an internal read-only memory and an internal rewriteable memory. A control program of the control unit is stored at least partially in the rewriteable memory. By re-programming the control program, it is theoretically possible in a controlled manner to change the controlled functions in the motor vehicle. For example, by manipulating the control program for the internal combustion engine, it is possible to increase the performance of the internal combustion engine in a relatively simple manner (so-called chip tuning). However, this often occurs at the cost of a long service life and of low exhaust emissions of the engine. For this reason, an unauthorized re-programming of the control program of a control unit renders liability and warranty claims null and void.
Familiar from the related art are various methods for protecting microcomputers against manipulation of the control program, and various protected microcomputers. In German Published Patent Application No. 197 23 332, the so-called seed & key method is described, which is widely used in the industry. In this known method, the validation program is stored in the internal read-only memory of the microcomputer. At every start-up of the microcomputer, a validation program is executed, in which, using a key, a code word is established from at least one part of the memory contents of the rewriteable memory and is compared with a comparison code word stored in the rewriteable memory. In the event that the code word agrees with the comparison code word, the microcomputer is enabled for executing further programs. Otherwise, the microcomputer is at least partially blocked.
If the control program of the microcomputer is manipulated, then, first, the rewriteable memory is erased and a manipulated control program is overwritten. In this context, the comparison code word originally stored in the rewriteable memory is lost. To generate a new code word, the key is required, which, however, is not readily available. Therefore, after a manipulation of the control program, the comparison code word and the code word usually do not agree, and the microcomputer is blocked.
However, the seed & key method known from the related art presupposes a microcomputer that has an internal read-only memory, in which the validation program is stored. The known method does not function in a microcomputer that does not have available to it an internal memory.